This invention relates generally to arc welding processes, and more particularly to a method of determining the weldability of a part.
The aerospace industry, among others, frequently uses "super" or "exotic" alloys for fabricating parts and components, which entails a need to weld such materials. Such superalloys include nickel, cobalt and iron based alloys which have high strength at high temperatures. For example, Inconel 718 alloy is a nickel based superalloy. These alloys are typically characterized by rather low concentrations of trace elements or impurities such as sulfur or oxygen. The specifications for a particular alloy usually set maximum limits for impurity concentrations, and the concentrations of impurities can vary between different heats or casts of the same alloy. These cast-to-cast variations in impurity concentrations can cause different heats of the same alloy to exhibit substantially different weldability.
Within certain limits of trace element concentrations, the weldability characteristics of an alloy may not be appreciably affected. However, at low impurity concentrations, minor variations in concentrations of some trace elements can result in substantial variations in weld penetration. With certain alloys such as Inconel 718 alloy or Type 300 stainless steels, the impurity concentrations in the alloy, especially sulfur, determine its weldability. For sulfur concentrations above about 100-150 ppm (parts per million) variations in sulfur concentration have little affect on weldability. However, for sulfur concentrations below about 50-60 ppm, minor variations in concentration can result in substantial variations in weld penetration for the same welding parameters. Reduced penetration may result from too low a concentration, and differences in weld penetration may not be apparent to the human or machine welder as changes in the face side weld pool features are not readily discernable. This has caused problems in welding parts made from such alloys. The typical approach is to weld the parts and inspect them afterwards. The disadvantages of this approach are apparent. The parts may have to be discarded or the welded joint may have to be reworked in order to obtain the specified penetration. Chemical analysis of the parts prior to welding may have to be performed if problems have been repeatedly encountered.
It is desirable to provide a method of determining the weldability of a part prior to welding which avoids the foregoing difficulties by enabling appropriate changes in welding parameters to be made prior to welding so that a welded joint having the desired characteristics is obtained. It is to this end that the present invention is principally directed.